1. Field of the Invention
The present invention relates to a fixing device for forming a toner image in an image forming apparatus for such as an electrophotographic apparatus or an electrostatic recording apparatus, and more particularly to such fixing device provided with a rotary member such as a roller or a belt.
2. Description of the Prior Art
The toner offsetting in a fixing device with a roller has conventionally been prevented by providing the fixing roller with a releasing surface layer composed for example of RTV silicone rubber or Teflon (trade name of du Pont de Nemeur) and eventually by coating a releasing agent such as silicone oil. A fixing roller surfacially covered with RTV silicone rubber permits simplification of the device as the coating of the releasing agent is generally unnecessary and provides satisfactory fixing ability owing to the elasticity of silicone rubber, but is inevitably associated with a very short service life, requiring frequent replacements because of rapid time-dependent deterioration of the releasing ability. On the other hand the releasing agent such as silicone oil, if applied for maintaining the releasing ability, swells the silicone rubber and deteriorates the mechanical strength thereof, thus leading again to frequent replacements of the roller.
In comparison with the silicone rubber coated fixing roller described above, a fixing roller surfacially coated with a fluorinated resin such as tetrafluoroethylene-fluoroalkoxyethylene copolymer (hereinafter called resin A), polytetrafluoroethylene resin (hereinafter called resin B) or Teflon (trade name of du Pont de Nemeur) has a higher strength and shows sufficient releasing ability if silicone oil is applied thereon.
Such roller is however associated with the disadvantages of lower fixing ability in comparison with the aforementioned silicone rubber coated roller and of crushing the image in fixing a toner image on a toner image bearing member thereby deteriorating the image quality, because of the lack of elasticity. Furthermore, in case a mechanical member such as a cleaning blade is maintained in contact with the roller, the roller may be easily damaged due to the lack of elasticity when a hard dust particle sticking on the roller intrudes between said member and the roller.
The above-mentioned improved fixing ability attained with an elastic member such as silicone rubber is presumably ascribable to a fact that the fixing roller can adapt itself to the profile of the toner image on the image bearing member, thus achieving uniform contact over the entire surface.
On the other hand it is estimated that a rigid roller composed for example of Teflon contacts strongly with the protruding area of the toner image and the bearing member but gives only an unstable contact with the recessed area thereof, thus resulting in a very insufficient image fixation in such recessed area. Experimental results show that a same toner image can be completely fixed at a lower temperature by 20.degree. to 50.degree. C. with silicone rubber rather than with Teflon.
Japanese Patent Laid-Open No. Sho 48-85151 discloses a fixing roller composed of a mixture of powder of tetrafluoroethylene resin and unvulcanized silicone rubber, but such roller is associated with the drawbacks of a very low mechanical strength because of poor compatibility, mutual dispersibility and adhesion between the silicone and the resin, and a significant loss of mechanical strength resulting from swelling of the silicone rubber when a releasing agent such as silicone oil is applied on the roller.